Earth tiebacks, also generally referred to as ground anchors, are often used in the construction industry to support or anchor various structures in the ground. For example, they are used to support retaining walls bordering highways or to support excavation sheeting to prevent cave-ins which would otherwise endanger lives and property. Such tiebacks generally comprise a prestressing steel tendon, fabricated from multi-element strands or a single bar, installed in the ground and secured at its outer end by an anchorhead to an excavation sheeting system or other structure to be supported. A cement grouted anchor is formed around the opposite distal end of the tendon to distribute to the surrounding soil forces applied to the tendon.
High load-carrying capacity tiebacks are made in coarse-grained soils by a variety of methods. Such high capacity tiebacks are installed by pumping cement grout under high pressure, for example, at a pressure above 150 psi, which anchors the prestressing steel (tieback tendon) to the ground. At such high grout pressure, water in the cement grout is quickly driven from the grout into the soil as the grout is pumped out the bottom of the casing. The cement particles in the grout remain creating very stiff grout surrounding the anchor tendon and the lower portion of the casing. This stiff, solidifying grout makes it difficult to withdraw the casing so that special techniques or methods have been developed to install high capacity tiebacks.
In one prior art method a drill is used to rotate the casing into the ground; and in another prior art method the casing is driven into the ground using a percussion hammer. The drilled casing uses a "lost" bit while the driven casing uses a "lost" point, attached to the front of the casing. After the casing has been installed to the desired depth, the prestressing steel is inserted into the casing and the bit or point is removed. Cement grout is then pumped down the casing while the casing is extracted with hydraulic jacks. The drills used to install tieback casings normally have about 4,000 to 6,000 ft.-lbs. of torque and 16,000 lbs. of pulling force. The casing-pulling jacks used to extract the casing must be capable of applying more than 50 tons of pulling force. These methods have the disadvantage of requiring two separate pieces of equipment, i.e., the dril or percussion hammer to install the casing and the jack and hydraulic power supply to withdraw the casing. Also, sectional casing must be used since the drills and percussion hammers are not capable of installing long continuous casing. Sectional casing is both more expensive and time consuming to use than a one piece casing. When sectional casing is used, bar tendons must be used because strand tendons have become grouted to the casing when high pressure is used to apply the grout.
Another method of installing high capacity tiebacks in sand has been attempted, but to date it has been unsuccessful. In this method the casing is drilled into the ground, to a desired depth. Thereafter, the prestressing steel is inserted into the casing. The anchor is grouted by pumping grout down the casing at high pressure as the drill attempts to extract the casing. In these attempts the drill has applied at torque of only approximately 6,000 ft.-lbs. This torque is not sufficient to overcome the friction developed between the casing and the grout placed under high pressure resulting in failure to extract the casing and thus a considerable amount of lost casing. This attempted method is therefore incapable of attaining high load-carrying capacity tiebacks.
A further prior method of installing a high-pressure-grouted anchor, disclosed in U.S. Pat. No. 3,494,134, consists of drilling a casing into the ground and inserting the prestressing steel along with a special grout pipe into the casing. In another prior technique, grout is placed around the prestressing steel under low pressure as the casing is extracted. The drill is capable of extracting the casing because the friction between the low pressure grout and the casing is low. After the initial low pressure applied grout has set (approximately 24 hours), grout is injected into the anchor zone portion of the tieback under high pressure through the special grout pipe. The high pressure grout fractures the low pressure grout surrounding the anchor. Then the tieback anchor is grouted under high pressure through the special grout pipe. This technique is time consuming since it requires two separate grout applications with an extended time interval between applications. An additional component, the special grout pipe, is also required, as well as a larger hole.